Trimboli is of Italian ancestry and attended Xavier College in Melbourne, where he was a member of the first soccer XI, captained by his brother.

In 1990/91, Trimboli helped South win their second NSL Championship – and their first since 1984 – by providing the all important assist for Joe Palatsides' last-minute equaliser in the Grand Final against Melbourne Knights.

In 1997/1998, Trimboli captained the Ange Postecoglou-led South Melbourne side to the Championship with a 2–1 win over Carlton in the Grand Final, whilst snaring his second Johnny Warren medal.

Trimboli repeated the heroics in 1998/1999 by contributing a goal in South Melbourne's 3–2 win in the Grand Final against Sydney United.

Trimboli led his Hellas team-mates against footballing giants Manchester United, Necaxa of Mexico and Brazil's Vasco Da Gama - all three of their matches taking place in the iconic Maracana stadium.